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Side by side saddle or piggy back scope alignment

 

This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.

Thanks

Chuck
 

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There’s no need for them to be perfectly parallel. ?If it weren’t for atmospheric refraction, flexure, and polar mis-alignment, you could simultaneously image two completely different parts of the sky. ?Given that those things do happen, you probably want the guide scope to be tightly coupled to the long-FL scope, and keep the short-FL scope pointed in roughly the same direction. ?Also, the go-to controller is only going to be aligned for one scope if they don’t point in the same direction.?

? -Les



On 10 Aug 2020, at 21:02, crocco1250 via <crocco1250@...> wrote:

This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.

Thanks

Chuck

 

Les

Thanks

Chuck


-----Original Message-----
From: Les Niles <les@...>
To: Losmandy_users <[email protected]>
Sent: Mon, Aug 10, 2020 09:47 PM
Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment


There’s no need for them to be perfectly parallel. ?If it weren’t for atmospheric refraction, flexure, and polar mis-alignment, you could simultaneously image two completely different parts of the sky. ?Given that those things do happen, you probably want the guide scope to be tightly coupled to the long-FL scope, and keep the short-FL scope pointed in roughly the same direction. ?Also, the go-to controller is only going to be aligned for one scope if they don’t point in the same direction.?

? -Les



On 10 Aug 2020, at 21:02, crocco1250 via <crocco1250@...> wrote:

This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.

Thanks

Chuck

Sonny Edmonds
 

I'm pretty anal, so I try and get everything .
I'm toying with the idea of dual telescopes, but pretty convinced I want a piggy back because of imaging straight through from one meridian into the other.
Side by side could prove more limiting than piggy backed (or over and under).

I put a Z-bolt laser on my rig yesterday, and zeroed in everything last night.
I now have 4 aiming options at my fingertips. Red Dot, Laser, Guide Scope, and Main Imaging. And they are coplaner. But subject to further refinements.
The laser proved it's worth during my modeling. I can't always do the contortions to look through my Red Dot to "rough in" a star.
But the thin green line let's me get the marks.
I like it when I can get instant worth from a product.

--
SonnyE


(I suggest viewed in full screen)
 

Chuck,

If I am reading your post correctly, you are wondering whether the scopes?should be parallel?or should point to the same place in the sky (I assume that is what you mean by having the views concentric). If they are parallel, they will point to the same place in the sky, despite the fact they are separated by several inches. This would NOT be true for a terrestrial object where the distance between two parallel scopes would offset the field of view by the same spacing as between the two optical axes.? For an astronomical object such as the moon, your best case resolution is going to be on the order of a kilometer or so, and you would never notice such a small offset. In similar fashion, you would be hard pressed to see the difference in the fields of view on a terrestrial object at a distance?of several miles. In those cases, the lines of sight for the two telescopes?are effectively parallel. Thus, two telescopes which are parallel will point to the same location on?the sky.

I hope I haven't misread your original question, if so I apologize. At any rate, I hope you find my reply useful.

Best regards,

Brett

On Tue, Aug 11, 2020 at 12:02 AM crocco1250 via <crocco1250=[email protected]> wrote:
This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.

Thanks

Chuck

 

Brett

Thanks for your reply. Yes, by concentric I meant having the same view in the sky. Since I put a losmandy dovetail on the top and bottom of my c11 (it has hyperstar capability) and put an 80 mm refractor f5 on top of that it was apparetly their view in the sky was close but not totally concentric and the refractor doesn't have adjustable rings. So i was wondering do I shim the dovetail using a laser to get the two perfectly parallel or not. I understood in general their view of the sky would be nearly the same but would that matter and in what circumstances.? But I found they were slightly off of each other.? Since I don't do AP yet but have been playing with a camera and I am a complete babe in the woods with AP I was looking for input.?

Chuck

-----Original Message-----
From: E. Brett Waller <cedargreenobservatory@...>
To: Losmandy_users <[email protected]>
Sent: Tue, Aug 11, 2020 01:25 PM
Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment


Chuck,

If I am reading your post correctly, you are wondering whether the scopes?should be parallel?or should point to the same place in the sky (I assume that is what you mean by having the views concentric). If they are parallel, they will point to the same place in the sky, despite the fact they are separated by several inches. This would NOT be true for a terrestrial object where the distance between two parallel scopes would offset the field of view by the same spacing as between the two optical axes.? For an astronomical object such as the moon, your best case resolution is going to be on the order of a kilometer or so, and you would never notice such a small offset. In similar fashion, you would be hard pressed to see the difference in the fields of view on a terrestrial object at a distance?of several miles. In those cases, the lines of sight for the two telescopes?are effectively parallel. Thus, two telescopes which are parallel will point to the same location on?the sky.

I hope I haven't misread your original question, if so I apologize. At any rate, I hope you find my reply useful.

Best regards,

Brett

On Tue, Aug 11, 2020 at 12:02 AM crocco1250 via <crocco1250=[email protected]> wrote:
This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.

Thanks

Chuck

 

Chuck,

Two scopes with the centerline of their optical axes exactly parallel will point to the same location on the sky. So if you shim them to be parallel. their fields of view will be concentric.

Brett?

On Tue, Aug 11, 2020 at 6:34 PM crocco1250 via <crocco1250=[email protected]> wrote:
Brett

Thanks for your reply. Yes, by concentric I meant having the same view in the sky. Since I put a losmandy dovetail on the top and bottom of my c11 (it has hyperstar capability) and put an 80 mm refractor f5 on top of that it was apparetly their view in the sky was close but not totally concentric and the refractor doesn't have adjustable rings. So i was wondering do I shim the dovetail using a laser to get the two perfectly parallel or not. I understood in general their view of the sky would be nearly the same but would that matter and in what circumstances.? But I found they were slightly off of each other.? Since I don't do AP yet but have been playing with a camera and I am a complete babe in the woods with AP I was looking for input.?

Chuck
-----Original Message-----
From: E. Brett Waller <cedargreenobservatory@...>
To: Losmandy_users <[email protected]>
Sent: Tue, Aug 11, 2020 01:25 PM
Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment


Chuck,

If I am reading your post correctly, you are wondering whether the scopes?should be parallel?or should point to the same place in the sky (I assume that is what you mean by having the views concentric). If they are parallel, they will point to the same place in the sky, despite the fact they are separated by several inches. This would NOT be true for a terrestrial object where the distance between two parallel scopes would offset the field of view by the same spacing as between the two optical axes.? For an astronomical object such as the moon, your best case resolution is going to be on the order of a kilometer or so, and you would never notice such a small offset. In similar fashion, you would be hard pressed to see the difference in the fields of view on a terrestrial object at a distance?of several miles. In those cases, the lines of sight for the two telescopes?are effectively parallel. Thus, two telescopes which are parallel will point to the same location on?the sky.

I hope I haven't misread your original question, if so I apologize. At any rate, I hope you find my reply useful.

Best regards,

Brett

On Tue, Aug 11, 2020 at 12:02 AM crocco1250 via <crocco1250=[email protected]> wrote:
This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.

Thanks

Chuck

 

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Chuck -- If you are wondering about the effect of misalignment of the guidescope on astrophotography, it can cause star trailing.? This is due to “field rotation” and it could occur if the polar alignment isn’t perfect, and the guidescope and OTA don’t point to the same spot in the sky. There is a good calculator available for this () and an article that explains it in some detail.? For exposures of a few minutes or less and decent polar alignment (as with Polemaster, Sharpcap Pro, or drift alignment), and reasonably close alignment of the OTA with the guidescope, it’s not likely to be a problem.? I aligned my OTA and guidescope on a tower about 3 miles away and I haven’t seen a problem in my images, but YMMV.?

?

Joe

?

From: [email protected] <[email protected]> On Behalf Of E. Brett Waller
Sent: Friday, August 21, 2020 2:30 AM
To: [email protected]
Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment

?

Chuck,

?

Two scopes with the centerline of their optical axes exactly parallel will point to the same location on the sky. So if you shim them to be parallel. their fields of view will be concentric.

?

Brett?

?

On Tue, Aug 11, 2020 at 6:34 PM crocco1250 via <crocco1250=[email protected]> wrote:

Brett

?

Thanks for your reply. Yes, by concentric I meant having the same view in the sky. Since I put a losmandy dovetail on the top and bottom of my c11 (it has hyperstar capability) and put an 80 mm refractor f5 on top of that it was apparetly their view in the sky was close but not totally concentric and the refractor doesn't have adjustable rings. So i was wondering do I shim the dovetail using a laser to get the two perfectly parallel or not. I understood in general their view of the sky would be nearly the same but would that matter and in what circumstances.? But I found they were slightly off of each other.? Since I don't do AP yet but have been playing with a camera and I am a complete babe in the woods with AP I was looking for input.?

Chuck
-----Original Message-----
From: E. Brett Waller <cedargreenobservatory@...>
To: Losmandy_users <[email protected]>
Sent: Tue, Aug 11, 2020 01:25 PM
Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment

Chuck,

?

If I am reading your post correctly, you are wondering whether the scopes?should be parallel?or should point to the same place in the sky (I assume that is what you mean by having the views concentric). If they are parallel, they will point to the same place in the sky, despite the fact they are separated by several inches. This would NOT be true for a terrestrial object where the distance between two parallel scopes would offset the field of view by the same spacing as between the two optical axes.? For an astronomical object such as the moon, your best case resolution is going to be on the order of a kilometer or so, and you would never notice such a small offset. In similar fashion, you would be hard pressed to see the difference in the fields of view on a terrestrial object at a distance?of several miles. In those cases, the lines of sight for the two telescopes?are effectively parallel. Thus, two telescopes which are parallel will point to the same location on?the sky.

?

I hope I haven't misread your original question, if so I apologize. At any rate, I hope you find my reply useful.

?

Best regards,

?

Brett

?

On Tue, Aug 11, 2020 at 12:02 AM crocco1250 via <crocco1250=[email protected]> wrote:

This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.

Thanks

Chuck

Arun Hegde
 

In reading the article linked (authored by Frank Barrett), I am not so sure that those calculations apply to the type of guided imaging we do. In fact, you could use his calculations to prove that with reasonable care, field rotation is unlikely to be a problem at all with today's methods of polar alignment.?

Hook's equations in Frank's article are for unguided imaging and Frank appears to have extended it to guided imaging. Which is fine, but then the relevant exposure time is not the exposure time of your sub, but the exposure time of your guide camera. This is normally on the order of a few seconds, and much shorter than the exposure time of your sub.? As Frank notes, in autoguiding, the motion of the centroid of the guide star is tracked with reference to a reference pixel. Motion beyond a certain tolerance results in a correction. And that correction is applied in both RA and DEC. And as long as that correction is applied before appreciable field rotation occurs, you should be fine. As noted by Frank himself in his example:

"In other words, if our alignment error is less than about 11.25 arc minutes we should see no field rotation greater than 9 microns during a 15-minute exposure at 35 degrees declination and the given setup."


11.25 arc minutes is an absolutely horrendous standard for polar alignment! We can easily get under 2 arc minutes, and with some care, under 1 arc minute in a matter of minutes using things like SharpCap. And guide exposures are much, much smaller than 15 minutes usually 2-4 seconds.

To the point then about the mismatch between the guide scope and the OTA. With reasonable polar alignment, the issue isn't field rotation, but atmospheric refraction. If the guide cam is pointing at a vastly different area of the sky refraction will cause differences in the motion of your subject versus guide star (eg. see discussion of King Rate versus Sidereal Rate). You should be able to avoid this very easily by even visually assuring that the guide cam and OTA are atleast somewhat aligned.?

?
 

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Good point. I was using total image exposure time and even there, field rotation wasn’t a problem I had to worry about.?

On Aug 21, 2020, at 12:16 PM, Arun Hegde <arun.k.hegde@...> wrote:

?In reading the article linked (authored by Frank Barrett), I am not so sure that those calculations apply to the type of guided imaging we do. In fact, you could use his calculations to prove that with reasonable care, field rotation is unlikely to be a problem at all with today's methods of polar alignment.?

Hook's equations in Frank's article are for unguided imaging and Frank appears to have extended it to guided imaging. Which is fine, but then the relevant exposure time is not the exposure time of your sub, but the exposure time of your guide camera. This is normally on the order of a few seconds, and much shorter than the exposure time of your sub.? As Frank notes, in autoguiding, the motion of the centroid of the guide star is tracked with reference to a reference pixel. Motion beyond a certain tolerance results in a correction. And that correction is applied in both RA and DEC. And as long as that correction is applied before appreciable field rotation occurs, you should be fine. As noted by Frank himself in his example:

"In other words, if our alignment error is less than about 11.25 arc minutes we should see no field rotation greater than 9 microns during a 15-minute exposure at 35 degrees declination and the given setup."


11.25 arc minutes is an absolutely horrendous standard for polar alignment! We can easily get under 2 arc minutes, and with some care, under 1 arc minute in a matter of minutes using things like SharpCap. And guide exposures are much, much smaller than 15 minutes usually 2-4 seconds.

To the point then about the mismatch between the guide scope and the OTA. With reasonable polar alignment, the issue isn't field rotation, but atmospheric refraction. If the guide cam is pointing at a vastly different area of the sky refraction will cause differences in the motion of your subject versus guide star (eg. see discussion of King Rate versus Sidereal Rate). You should be able to avoid this very easily by even visually assuring that the guide cam and OTA are atleast somewhat aligned.?

?
Arun Hegde
 

The linked document is very nice though.

You can use it to prove to yourself that the angle between the guide camera and main scope is largely irrelevant at least from a polar alignment standpoint as in my example below.?

Assume that the max tolerance for field rotation is 3.76 microns (typical pixel size), target declination is 85 degrees (close to the worst case scenario), polar alignment is within 3 arc minutes (we can easily do better than that), guide exposure is 4 seconds, and focal length is 3000 mm (most of us image considerably shorter focal lengths, and anyone imaging at this long of a focal length is likely using an off axis guider).?

Then the maximum angle between the guide scope and main OTA is 25 degrees to prevent visible field rotation in 4 seconds. You would easily detect this mismatch just visually looking at your setup. No optical alignment needed.

For more typical cases (eg. 600mm f/l, smaller declinations), the permissible deviations are even higher.?

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